The present invention relates to a structure of a threaded paper tube which is freely changeable of its total length, simple and easy to manufacture and also readily disposable by burning.
A screwing structure conventionally widely known is to use a tubular member having thread grooves on its inner surface and a corresponding tubular member or a cylindrical member having thread grooves on its outer surface, typically such as a combination of a bolt and a nut. Furthermore, there has been such a concept that an elongated nut set in length larger than usual ones may be used to enable adjustment of the total length of the whole structure by means of the extent of screwing between the nut and bolt unless the end portion of the bolt extends out of the nut. But, the feature is hitherto seen and adopted (thereby not novel) in the structure of "turnbuckle" or that of an adjustment mechanism for adjusting legs length of machines or tables to level them.
The conventional screwing features have been applied, for example, in a table with four legs, in such a manner that female screws are burried at lower ends of the table legs and male screws having flanges are screwed with the female screws, in order to provide fine adjustment of length of the table legs for enabling the table to be levelled of its surface or to be eliminated of shakiness.
The conventional features have the following defects. In case of using the screws of larger diameter, they inherently and necessarily become much larger in weight. And the screws cannot be set in pitches to be substantially large, thereby be not capable of having a large extent of change of the total length. Furthermore, the screws are not easy to produce, whereby necessitating high cost to produce.
In the meantime, a spiral-winding paper tube has been often used as a winding core for synthetic fiber yarn or as a trunk portion of a cylindrical container. Such paper tubes are manufactured basically in such manner that a material paper in a ribbon-like shape is wound continuously and spirally on a mandrel used as a core while the material being adhered by use of an adhesive, thereafter to be removed from the mandrel. The features of the foregoing screws and the feature of the abovesaid spiral-winding paper tube are different at all in the manufacturing methods and shapes but are identical to each other as having the spiral portions.